Well... if you take the implications of SSJ Kid Trunks < Base Future Gohan seriously, then pretty damn weak. But in general I think the Namek tier of Base power is around what it takes. Vegeta thought even Goku at 60,000 could do it, but that was just guesswork.

Absolute Zamasu(my name for their fusion) wins. He fulfils his utopia and dwells in his loneliness(x amount of time for this to happen). He starts to doubt of his achievements, Goku's body influences him to seek out challenges but there's nothing. Perfection becomes a curse. Absolute Zamasu starts to dread his existence and presents himself to the Omni-King, detailing on his actions.
Using his powers the Omni-King restores everything to the moment before Goku and Vegeta perish, humanity's last barrier. Zamasu concedes. He ask for forgiveness(mainly to Goku, Gowasu and Trunks) and understands that his justice isn't justice at all. He asks for judgement and the Omni-King delivers.
Trunks is forced never to travel in time ever again, all the time-machines are destroyed.